To meet the demands of the exponential growth in video, voice, data, and mobile-device traffic over the Internet, the telecommunication industry has been moving toward 40-Gbps and 100-Gbps data rates for their core fiber-optic backbone networks alongside the existing 10-Gbps infrastructure, which operates at the low-loss wavelength window of 1.55 μm. Many of the modulation techniques that are effective at these high speeds (for example, being tolerant to polarization-mode dispersion, chromatic dispersion, and intersymbol interference (ISI), as well as having competitive edge in price and performance combination) require receivers that are based upon the direct detection of optical pulses. These techniques include phase-shaped binary transmission (PSBT) or duo-binary modulation (DM), return-to-zero on-off-keying (RZ-OOK), and carrier-suppressed return-to-zero (CS-RZ). Typically, such high-speed operation uses high sensitivity detectors.
Separate absorption and multiplication (SAM) InP-InGaAs avalanche photodiodes (APDs) are normally the most preferred photodetectors for direct-detection high-data rate systems for two main reasons. First, they have high sensitivity, which results from the internal gain they generate. The gain is the total number of carriers generated through an avalanche of impact ionizations in response to a single carrier excitation. Second, they are highly cost effective compared to receivers that employ optical pre-amplification. Indeed, SAM InP-InGaAs APDs have been deployed in support of the synchronous optical networking (SONET) standards of OC-48 and OC-192, which operate at 2.5 Gbps and 10 Gbps, respectively. However, the long avalanche buildup time in InP, which is the time needed for all the impact ionizations to settle, has limited the speed of InP-based APDs and stopped them from meeting the expectations of 40-Gbps systems. There are few or no commercial APDs available for 40-Gbps communication, despite numerous efforts in the past two decades or so which targeted new APD materials and structures.
Viable options for direct detection of 40-Gbps bit streams include InGaAs PIN photodiodes, since very high bandwidths can be achieved with such state-of-the-art PIN photodiodes. However, PIN photodiodes have lower sensitivity than APDs since they do not offer any internal gain. To detect weak signals in the increased presence of Johnson noise in high data-rate systems, where increasing the bit rate by a factor of four causes the Jonson noise to increase by 6 dB, erbium doped fiber amplifiers (EDFAs) are typically used to pre-amplify the signals optically before their detection by the PIN photodiode. The resulting EDFA-PIN receiver can exhibit very high sensitivity (<−30 dBm), due to EDFA's high gain and low noise, as well as high speed, which is due to the high bandwidth of the PIN photodiode. However, these receivers can be bulky and expensive. An EDFA uses meters of fiber, which are generally coiled in a fairly sizeable disk, and more importantly, it uses a pump laser, which provides the optical amplification. For example, a 40-Gbps EDFA-InGaAs-based receiver module may cost up to $5,000. In contrast, APD-based receivers, which run for about $500, benefit from small form-factor packaging, since they can easily be integrated with the electronic components of the receiver circuit, and would offer a much more cost-effective solution than the EDFA-PIN receiver only if their speed were to be improved. Since ultra-low-noise APDs have already been demonstrated, the persisting challenge is the development of 1.55 μm APDs that can reliably offer gain-bandwidth-products (GBPs) in excess of 350 GHz, for example offering a gain of 10 at a speed of 35 GHz.
The long avalanche buildup time in InP is due to its roughly equal electron and hole ionization rates, where the hole-to-electron ionization coefficient ratio, k, is in the range 2.5-4 for InP. Moreover, the buildup time scales with the gain. Specifically, the buildup time limits the bandwidth at values of the gain (>10) that are useful in Johnson noise suppression. FIG. 1 shows a schematic illustrating the cascade of impact ionizations and the associated buildup time in a multiplication region of an APD such as an InP APD. Specifically, as a parent hole 102 is generated in an absorption layer (not shown), such as an InGaAs absorption layer, and injected in the InP multiplication region (parent hole 102 is shown at the bottom of the structure), a first wave of impact ionizations take place. The parent hole injected at the bottom initiates the avalanche. While the offspring holes drift together and reach the end of the multiplication region, the offspring electrons, which will still be present in the multiplication region, move in the opposite direction, as shown in the FIG. 1, causing a second-wave of impact ionizations. After the second wave ends, a third wave is launched, and so on, until eventually the impact ionization process ceases when all charge carriers exit the depletion layer, which is an intrinsic layer. The avalanche process includes hole ionization 101 and electron ionization 103. These ionizations provide a pulse 107 due to holes and a pulse 109 due to electrons, where the two pulses provide a total avalanche pulse 108. Number, locations, and times of impact ionizations are random, and the resulting avalanche pulse, where each drifting carrier contributes to the avalanche pulse, is therefore stochastic, with area representing the stochastic gain. The first wave refers to the carriers born in the first electron transit time, the second wave refers to those created in the hole transit time following the first electron transit time, and so on.
Indeed, the GBP of InGaAs/InP APDs has been limited to 170 GHz, which corresponds to a gain value below 5 if the bandwidth is constrained to a minimum value of 35 GHz in support of 40 Gbps bit rates. With such low gain, the APD cannot compete with the much faster PIN photodiodes combined with a high-gain (˜20 dB) EDFA.
There have been numerous efforts in the past two decades to explore new materials or new device concepts to overcome current limitations of InP and InAlAs APDs. For example, there have been efforts to engineer the multiplication region to minimize multiplication noise and maximize the GBP. Of particular importance is the discovery in the mid-1990s that submicron scaling of the multiplication region thickness leads to lower multiplication noise and higher gain-bandwidth products. In fact, the highest value of a commercial InGaAs/InP APD, 170 GHz, utilizes a very thin InP multiplication layer of 80 nm. Other efforts involved impact-ionization engineering of heterojunction multiplication regions, edge-illuminated and evanescently coupled waveguide structures, and use of In0.52Al0.48As material for multiplication. The k factor in In0.52Al0.48As is k=4-6.7, which gives it an edge over InP in terms of noise and GBP. InAlAs-based APDs have showed GBPs ranging from 70 to 170 GHz with the exception of two reported results with values of 290 GHz in 2000 and 320 GHz in 2001. It is now accepted that while both InP and InAlAs APDs have sufficient bandwidth to support 10-Gbps transmission, they cannot sustain gain-bandwidth tradeoff for 40 Gbps. A sensitivity of −19 dBm at 40 Gbps with a bit error rate of 10−10 has been demonstrated, providing approximately a 9 dB improvement over conventional PIN diode. This sensitivity was achieved by including a transimpedance amplifier with tunable response to boost the GBP from 140 to 270 GHz and by operating the APDs with avalanche gain values of 3 to 10. More recently, it was reported that that a Ge absorption layer grown directly on a Si multiplication layer provided a GBP of 340 GHz. This high GBP was attributed to the favorable ionization properties of Si. Most recently, Si on Ge APD with a GBP of 840 GHz operating at 1.31 μm was demonstrated. Despite all these efforts and advances, to this date there does not appear to be a commercial APD available to detect 40 Gbps signal, and the challenge is even greater for systems operating at 1.55 μm.